What is the independent variable

Read the excerpt from "mending wall." we keep the wall between us as we go. to each the boulders that have fallen to each. and some are loaves and some so nearly balls we have to use a spell to make them balance: "stay where you are until our backs are turned! " we wear our fingers rough with handling them. oh, just another kind of out-door game, one on a side. it comes to little more: there where it is we do not need the wall: he is all pine and i am apple orchard. what does the line “and some are loaves and some so nearly balls” refer to?